As a spray nozzle of this kind, the present applicant proposed a nozzle 100 shown in FIGS. 9(A) through 9(C), as disclosed in U.S. Pat. No. 2,719,073. Along the central axis L of a nozzle body 101, the nozzle 100 is provided with a main hole 102 serving as the gas-liquid mixing flow path for mixing water and compressed air with each other. The arc-shaped injection side front end of the lower hole portion 102a of the main hole 102 is formed proximately to the injection side end surface 101f of the nozzle body 101. A cut 104 diametrically formed on the injection side end surface 101f is communicated with the injection side front end portion of the lower hole portion 102a to form an oblong injection port 105. Sectionally circular auxiliary holes 106, 107 are formed at both sides of the lower hole portion 102a in the width direction thereof.
In the nozzle 100, owing to the construction in which the auxiliary holes 106, 107 are formed at both sides of the main hole 102, the gas-liquid mixture fluid which flows to both sides of the main hole 102 from the auxiliary holes 106, 107 is allowed to collide with the gas-liquid mixture fluid which flows along the central axis L of the main hole 102 so that the gas-liquid mixing is accelerated and the spray is homogenized. Thereby when the flow rate of water is low, it is possible to widen the spray angle. When the flow rate of the water is high, it is possible to restrain the spray angle from widening. Further even when the supply amount of the water is changed, it is possible to keep the spray angle approximately uniform.
Consequently, even though the supply amount of the water is changed with respect to a constant supply amount of compressed air, it is possible to keep the spray angle range, the flow rate distribution, the hitting power distribution, and the particle diameter uniformly. Thereby it is possible to uniformly cool slab by controlling the spray operation of the nozzle. This is attributed to an increased turndown ratio of 1:20. For example, the supply amount of water can be controlled in the range of 2 to 40 liters/minute with respect to a constant supply amount of compressed air constantly supplied at 0.4 NL/minute. By increasing the turndown ratio, it is possible to cool slab disposed in the range from the upstream region of the secondary cooling zone where it is necessary to supply a large amount of cooling water to the downstream region thereof where a small amount of the cooling water is sufficient for cooling the slab by using the same nozzle (nozzles), even though the thicknesses of the slab vary.